1. Field of the Invention
The present invention relates to a laser source module that aligns laser beams from a plurality of laser sources on a single optical axis for laser beam emission and a scanning image display device that displays an image on screen by two-dimensionally scanning a laser beam from the laser source module with a scanning mirror.
2. Description of the Related Art
Lately, developing small projectors that are handily portable and can display on a large screen is actively pursued. Small projectors that can be connected to a notebook PC or the like and video cameras with a built-in projector that can project recorded images, among others, are commercially available. Such a projector built in a mobile phone or a smart phone is expected to appear from here on.
Projectors of a preceding type use lamps or LEDs as light sources and project an image displayed by a liquid crystal panel or digital micro mirror device (DMD). However, development of laser projectors (scanning image display devices) is underway that use lasers as light sources and scan a single laser beam with movable mirrors to make a display. Because of using laser lights as the light sources, no focusing is needed. The laser projectors are considered to be fit for usage in which a user projects an image on a suitable wall anywhere he or she visits.
In Japanese Translation of PCT International Application Publication No. 2009-533715 (Patent Document 1), there is a description about a configuration of a scanning image display device capable of displaying a color image. This scanning image display device uses laser sources for three colors, i.e., red, blue, and green, and includes a beam combining unit that combines laser beams of the three colors into a combined beam traveling along a single axis and a beam scanner that scans the combined beam in deflection directions.
The beam combining unit is configured such that three laser sources arranged in parallel emit beams in a same direction and the beams are reflected by their respective beam combining mirrors and combined into a combined beam. An alternative configuration is also described in which one laser source is initially configured to emit a beam along the optical axis of the combined beam and only two beam combining mirrors are provided. It can be considered that a smaller module is possible because of fewer mirrors.
Since there was no laser that radiates a green light directly until quite recently, in Patent Document 1, a green light is still produced by wavelength conversion of an infrared light using SHG (second harmonic generation). However, even a laser that radiates a green light directly has been available lately.
For the scanning image display devices as mentioned above, it is important to align the optical axes of the laser beams of the three colors precisely. Misalignment or decentering of the optical axes gives rise to relative displacements of the spots of the respective colors on screen, resulting in a blurred image. Module assembly work entails making an adjustment to align the optical axes of the laser beams of the three colors.
Moreover, when a scanning image display device is used, its temperature rises mainly due to heat generated by the lasers, causing thermal deformation. Therefore, considerations also need to be taken for decentering of the optical axes due to the displacements of optical components when thermally deformed.
As a laser that is used as a light source, a cylindrical metal package product called a CAN package prevails. This product has a structure in which a half-cylindrical heat sink is coupled onto the front face of a cylindrical base, a laser chip is joined to the planar face of the heat sink with intermediate positioning of a sub-mount, and this assembly is covered by a cap.
As described in Japanese Published Unexamined Patent Application No. Hei 6-314857 (Patent Document 2), such a laser source suffers from a problem in which an emission point is displaced due to deformation by temperature rise. This is due to the fact that the base and heat sink have different coefficients of thermal expansion or the laser chip, sub-mount, and heat sink have different coefficients of thermal expansion, thus causing uneven thermal deformations of these elements. Since the direction of the optical axis of a laser beam from a laser source corresponds to the direction of a line connecting the emission point of the laser and the center of its lens, the optical axis of the laser beam is decentered when the emission point is displaced.
Patent Document 2 discloses a structure in which a cylindrical joint is used to join the heat sink to the base, thus preventing the emission point displacement as above. However, such a structure is not adopted in easily available laser products at present and the problem of the emission point displacement remains.
An object of the present invention is to provide a small laser source module capable of decreasing relative displacements of spots on screen even in the occurrence that the emission points of lasers are displaced because of temperature rise and a scanning image display device equipped with such module.